Divergent Mechanisms of Paraquat, MPP+, and Rotenone Toxicity: Oxidation of Thioredoxin and Caspase-3 Activation

Pharmacokinetic, neurochemical, stereological and neuropathological studies on the potential effects of paraquat in the substantia nigra pars compacta and striatum of male C57BL/6J mice

The pharmacokinetics and neurotoxicity of paraquat dichloride (PQ) were assessed following once weekly administration to C57BL/6J male mice by intraperitoneal injection for 1, 2 or 3 weeks at doses of 10, 15 or 25 mg/kg/week. Approximately 0.3% of the administered dose was taken up by the brain and was slowly eliminated, with a half-life of approximately 3 weeks. PQ did not alter the concentration of dopamine (DA), homovanillic acid (HVA) or 3,4-dihydroxyphenylacetic acid (DOPAC), or increase dopamine turnover in the striatum. There was inconsistent stereological evidence of a loss of DA neurons, as identified by chromogenic or fluorescent-tagged antibodies to tyrosine hydroxylase in the substantia nigra pars compacta (SNpc). There was no evidence that PQ induced neuronal degeneration in the SNpc or degenerating neuronal processes in the striatum, as indicated by the absence of uptake of silver stain or reduced immunolabeling of tyrosine-hydroxylase-positive (TH(+)) neurons. There was no evidence of apoptotic cell death, which was evaluated using TUNEL or caspase 3 assays. Microglia (IBA-1 immunoreactivity) and astrocytes (GFAP immunoreactivity) were not activated in PQ-treated mice 4, 8, 16, 24, 48, 96 or 168 h after 1, 2 or 3 doses of PQ. In contrast, mice dosed with the positive control substance, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 10mg/kg/dose×4 doses, 2 h apart), displayed significantly reduced DA and DOPAC concentrations and increased DA turnover in the striatum 7 days after dosing. The number of TH(+) neurons in the SNpc was reduced, and there were increased numbers of degenerating neurons and neuronal processes in the SNpc and striatum. MPTP-mediated cell death was not attributed to apoptosis. MPTP activated microglia and astrocytes within 4 h of the last dose, reaching a peak within 48 h. The microglial response ended by 96 h in the SNpc, but the astrocytic response continued through 168 h in the striatum. These results bring into question previous published stereological studies that report loss of TH(+) neurons in the SNpc of PQ-treated mice. This study also suggests that even if the reduction in TH(+) neurons reported by others occurs in PQ-treated mice, this apparent phenotypic change is unaccompanied by neuronal cell death or by modification of dopamine levels in the striatum.

Differential Effects of Methyl-4-Phenylpyridinium Ion, Rotenone, and Paraquat on Differentiated SH-SY5Y Cells

Paraquat (PQ), a cationic nonselective bipyridyl herbicide, has been used as neurotoxicant to modulate Parkinson's disease in laboratory settings. Other compounds like rotenone (ROT), a pesticide, and 1-methyl-4-phenylpyridinium ion (MPP(+)) have been widely used as neurotoxicants. We compared the toxicity of these three neurotoxicants using differentiated dopaminergic SH-SY5Y human cells, aiming to elucidate their differential effects. PQ-induced neurotoxicity was shown to be concentration and time dependent, being mitochondrial dysfunction followed by neuronal death. On the other hand, cells exposure to MPP(+) induced mitochondrial dysfunction, but not cellular lyses. Meanwhile, ROT promoted both mitochondrial dysfunction and neuronal death, revealing a biphasic pattern. To further elucidate PQ neurotoxic mechanism, several protective agents were used. SH-SY5Y cells pretreatment with tiron (TIR) and 2-hydroxybenzoic acid sodium salt (NaSAL), both antioxidants, and N ω -nitro-L-arginine methyl ester hydrochloride (L-NAME), a nitric oxide synthase inhibitor, partially protected against PQ-induced cell injury. Additionally, 1-(2-[bis(4-fluorophenyl)methoxy]ethyl)-4-(3-phenyl-propyl)piperazine (GBR 12909), a dopamine transporter inhibitor, and cycloheximide (CHX), a protein synthesis inhibitor, also partially protected against PQ-induced cell injury. In conclusion, we demonstrated that PQ, MPP(+), and ROT exerted differential toxic effects on dopaminergic cells. PQ neurotoxicity occurred through exacerbated oxidative stress, with involvement of uptake through the dopamine transporter and protein synthesis.

Thioredoxin-mediated redox regulation of resistance to endocrine therapy in breast cancer

Resistance to endocrine therapy in breast carcinogenesis due to the redox regulation of the signal transduction system by reactive oxygen species (ROS) is the subject of this review article. Both antiestrogens and aromatase inhibitors are thought to prevent cancer through modulating the estrogen receptor function, but other mechanisms cannot be ruled out as these compounds also block metabolism and redox cycling of estrogen and are free radical scavengers. Endocrine therapeutic agents, such as, tamoxifen and other antiestrogens, and the aromatase inhibitor, exemestane, are capable of producing ROS. Aggressive breast cancer cells have high oxidative stress and chronic treatment with exemestane, fulvestrant or tamoxifen may add additional ROS stress. Breast cancer cells receiving long-term antiestrogen treatment appear to adapt to this increased persistent level of ROS. This, in turn, may lead to the disruption of reversible redox signaling that involves redox-sensitive phosphatases, protein kinases, such as, ERK and AKT, and transcription factors, such as, AP-1, NRF-1 and NF-κB. Thioredoxin modulates the expression of estrogen responsive genes through modulating the production of H2O2 in breast cancer cells. Overexpressing thioredoxine reductase 2 and reducing oxidized thioredoxin restores tamoxifen sensitivity to previously resistant breast cancer cells. In summary, it appears that resistance to endocrine therapy may be mediated, in part, by ROS-mediated dysregulation of both estrogen-dependent and estrogen-independent redox-sensitive signaling pathways. Further studies are needed to define the mechanism of action of thioredoxin modifiers, and their effect on the redox regulation that contributes to restoring the antiestrogen-mediated signal transduction system and growth inhibitory action.

Neuroprotective mechanisms of the standardized extract of Bacopa monniera in a paraquat/diquat-mediated acute toxicity

Parkinson's disease (PD) is one of the most common age related neurodegenerative disease and affects millions of people worldwide. Strong evidence suggests a role for oxidative stress and mitochondrial dysfunctions in the pathogenesis of PD. Recent epidemiologic and toxicological studies have shown that environmental factors, especially herbicides such as paraquat and diquat represent one of the primary classes of neurotoxic agents associated with PD. The objective of our study was to investigate the neuroprotective effects of the standardized extract of Bacopa monniera (BM) against paraquat/diquat-induced toxicity and to elucidate the mechanisms underlying this protection. Our results showed that a pre-treatment with the BM extract, from 20.0μg/ml, protected the rat dopaminergic PC12 cell line against paraquat/diquat-induced toxicity in various cell survival assays. We demonstrated that BM pre-treatment, from 5.0μg/ml, could prevent the generation of intracellular reactive oxygen species (ROS), decreased mitochondrial superoxide levels and depolarized the mitochondria. BM pre-treatment also increased tyrosine hydroxylase (TH) levels and antioxidant defense systems such as γ-glutamylcysteine synthetase (γ-GCS) and thioredoxin1 (Trx1) levels. Furthermore, BM pre-treatment prevented the activation of Akt and heat shock protein90 (HSP90) proteins. Thus, our findings demonstrated that BM can protect PC12 cells through modulating cellular redox pathways which are altered in PD and could have a therapeutic application in the prevention of PD.

Glutaredoxin 1 protects dopaminergic cells by increased protein glutathionylation in experimental Parkinson's disease

AIMS

Chronic exposure to environmental toxicants, such as paraquat, has been suggested as a risk factor for Parkinson's disease (PD). Although dopaminergic cell death in PD is associated with oxidative damage, the molecular mechanisms involved remain elusive. Glutaredoxins (GRXs) utilize the reducing power of glutathione to modulate redox-dependent signaling pathways by protein glutathionylation. We aimed to determine the role of GRX1 and protein glutathionylation in dopaminergic cell death.

RESULTS

In dopaminergic cells, toxicity induced by paraquat or 6-hydroxydopamine (6-OHDA) was inhibited by GRX1 overexpression, while its knock-down sensitized cells to paraquat-induced cell death. Dopaminergic cell death was paralleled by protein deglutathionylation, and this was reversed by GRX1. Mass spectrometry analysis of immunoprecipitated glutathionylated proteins identified the actin binding flightless-1 homolog protein (FLI-I) and the RalBP1-associated Eps domain-containing protein 2 (REPS2/POB1) as targets of glutathionylation in dopaminergic cells. Paraquat induced the degradation of FLI-I and REPS2 proteins, which corresponded with the activation of caspase 3 and cell death progression. GRX1 overexpression reduced both the degradation and deglutathionylation of FLI-I and REPS2, while stable overexpression of REPS2 reduced paraquat toxicity. A decrease in glutathionylated proteins and REPS2 levels was also observed in the substantia nigra of mice treated with paraquat.

INNOVATION

We have identified novel protein targets of glutathionylation in dopaminergic cells and demonstrated the protective role of GRX1-mediated protein glutathionylation against paraquat-induced toxicity.

CONCLUSIONS

These results demonstrate a protective role for GRX1 and increased protein glutathionylation in dopaminergic cell death induced by paraquat, and identify a novel protective role for REPS2.

Thiol-redox signaling, dopaminergic cell death, and Parkinson's disease

SIGNIFICANCE

Parkinson's disease (PD) is characterized by the selective loss of dopaminergic neurons of the substantia nigra pars compacta, which has been widely associated with oxidative stress. However, the mechanisms by which redox signaling regulates cell death progression remain elusive.

RECENT ADVANCES

Early studies demonstrated that depletion of glutathione (GSH), the most abundant low-molecular-weight thiol and major antioxidant defense in cells, is one of the earliest biochemical events associated with PD, prompting researchers to determine the role of oxidative stress in dopaminergic cell death. Since then, the concept of oxidative stress has evolved into redox signaling, and its complexity is highlighted by the discovery of a variety of thiol-based redox-dependent processes regulating not only oxidative damage, but also the activation of a myriad of signaling/enzymatic mechanisms.

CRITICAL ISSUES

GSH and GSH-based antioxidant systems are important regulators of neurodegeneration associated with PD. In addition, thiol-based redox systems, such as peroxiredoxins, thioredoxins, metallothioneins, methionine sulfoxide reductases, transcription factors, as well as oxidative modifications in protein thiols (cysteines), including cysteine hydroxylation, glutathionylation, and nitrosylation, have been demonstrated to regulate dopaminergic cell loss.

FUTURE DIRECTIONS

In this review, we summarize major advances in the understanding of the role of thiol-redox signaling in dopaminergic cell death in experimental PD. Future research is still required to clearly understand how integrated thiol-redox signaling regulates the activation of the cell death machinery, and the knowledge generated should open new avenues for the design of novel therapeutic approaches against PD.

Preparation of acrylic acid-modified chitin improved by an experimental design and its application in absorbing toxic organic compounds

Chitin grafted poly (acrylic acid) (chi-g-PAA) is synthesized and characterized as an adsorbent of toxic organic compounds. Chi-g-PAA copolymers are prepared using of ammonium cerium (IV) nitrate (Ce(4+)) as the initiator. The highest grafting percentage of AA in chitin obtained using the traditional technique is 163.1%. A maximum grafting percentage of 230.6% is obtained using central composite design (CCD). Experimental results are consistent with theoretical calculations. The grafted copolymer is characterized by Fourier transform Infrared spectroscopy and solid state (13)C NMR. A representative chi-g-AA copolymer is hydrolyzed to a type of sodium salt (chi-g-PANa) and used in the adsorption of malachite green (MG), methyl violet (MV), and paraquat (PQ) in aqueous. The monolayer adsorption capacities of these substances are 285.7, 357.1, and 322.6 mg/g-adsorbent, respectively. Thermodynamic calculations show that the adsorption of MG, MV, and PQ are more favored at diluted solutions. The high adsorption capacity of chi-g-PANa for toxic matter indicates its potential in the treatment of wastewater and emergency treatment of PQ-poisoned patients.

Mitochondrial complex I inhibitor rotenone-induced toxicity and its potential mechanisms in Parkinson's disease models

The etiology of Parkinson's disease (PD) is attributed to both environmental and genetic factors. The development of PD reportedly involves mitochondrial impairment, oxidative stress, α-synuclein aggregation, dysfunctional protein degradation, glutamate toxicity, calcium overloading, inflammation and loss of neurotrophic factors. Based on a link between mitochondrial dysfunction and pesticide exposure, many laboratories, including ours, have recently developed parkinsonian models by utilization of rotenone, a well-known mitochondrial complex I inhibitor. Rotenone models for PD appear to mimic most clinical features of idiopathic PD and recapitulate the slow and progressive loss of dopaminergic (DA) neurons and the Lewy body formation in the nigral-striatal system. Notably, potential human parkinsonian pathogenetic and pathophysiological mechanisms have been revealed through these models. In this review, we summarized various rotenone-based models for PD and discussed the implied etiology of and treatment for PD.

Oxidative and apoptotic effects of lambda-cyhalothrin modulated by piperonyl butoxide in the liver of Oreochromis niloticus

The aim of this study was to investigate the toxic effects of pyrethroid pesticide lambda-cyhalothrin in the presence of piperonyl butoxide as a modulator in the liver of juvenile Oreochromis niloticus. LC(50) (96h) value of lambda-cyhalothrin was determined as 2.901μg/L for O. niloticus. The fish were exposed to 0.48μg/L (1/6 of the 96-h LC(50)) lambda-cyhalothrin and 10μg/L piperonyl butoxide for 96-h and 15-d. tGSH, GSH, GSSG, Hsp70 and TBARS contents, GPx, GR, GST and caspase-3 enzymes activities were determined. Lambda-cyhalothrin caused increases in tGSH, GSH, TBARS contents, and GST activity. Piperonyl butoxide treatment with lambda-cyhalothrin caused significant increases in tGSH GSH, Hsp70, TBARS contents, and GPx and GST activities while caspase-3 activity was decreased. The results of the present study revealed that lambda-cyhalothrin caused oxidative stress which upregulated GSH and GSH-related enzymes. Piperonyl butoxide increased the oxidative stress potential and apoptotic effects of lambda-cyhalothrin.

Paraquat-induced convulsion and death: a report of five cases

Paraquat (PQ) is a potent toxicant for humans, and poisoning with PQ is associated with high mortality. Patients with severe PQ-induced poisoning may die of multiple organ failure involving the circulatory and respiratory systems. Death resulting from epilepsy-like convulsions, which are infrequently noted reported with PQ poisoning, is observed clinically with this condition. This study presents the clinical data of five patients with severe PQ-induced poisoning who died of epilepsy-like convulsions, and related publications were reviewed in order to investigate the pathogenesis, clinical manifestations, and prognosis of these convulsions. Our results may help prevent this event and improve the success of treatment.

Minocycline prevents paraquat-induced cell death through attenuating endoplasmic reticulum stress and mitochondrial dysfunction

Paraquat (PQ) was demonstrated to induce dopaminergic neuron death and is used as a Parkinson's disease (PD) mimetic; however, its mechanism remains contradictory. Alternatively, minocycline is a second-generation tetracycline and is undergoing clinical trials for treating PD with an unresolved mechanism. We thus investigated the molecular mechanism of minocycline in preventing PQ-induced cytotoxicity. In this study, minocycline was effective in preventing PQ-induced apoptotic cell death, which involves the cleavages of poly (ADP-ribose) polymerase (PARP) and caspase 3 and increased fluorescence intensity of annexin V-FITC. In addition, PQ also quickly induced alterations of unfolded protein responses (UPRs) and subsequently dysfunction of the mitochondria (such as the decrease in membrane potential and increase in membrane permeability and superoxide formation). Finally, the mechanism of minocycline in preventing PQ-induced apoptosis might be mediated by attenuating endoplasmic reticulum (ER) stress and mitochondrial dysfunction, which respectively results in caspase-12 activation and the release of H2O2, HtrA2/Omi, and Smac/Diablo. Thus, minocycline could possibly be used to treat other neurodegenerative disorders with similar pathologic mechanisms.

Paraquat- and rotenone-induced models of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder mainly characterized by a loss of dopaminergic (DA) neurons in the substantia nigra pars compacta. In recent years, several new genes and environmental factors have been implicated in PD, and their impact on DA neuronal cell death is slowly emerging. However, PD etiology remains unknown, whereas its pathogenesis begins to be clarified as a multifactorial cascade of deleterious factors. Recent epidemiological studies have linked exposure to environmental agents, including pesticides, with an increased risk of developing the disease. As a result, over the last two decades the "environmental hypothesis" of PD has gained considerable interest. This speculates that agricultural chemicals in the environment, by producing selective dopaminergic cell death, can contribute to the development of the disease. However, a causal role for pesticides in the etiology of PD has yet to be definitively established. Importantly, most insights into PD pathogenesis came from investigations performed in experimental models of PD, especially those produced by neurotoxins. This review presents data obtained in our laboratories along with current views on the neurotoxic actions induced by the two most popular parkinsonian pesticide neurotoxins, namely paraquat and rotenone. Although confined to these two chemicals, mechanistic studies underlying dopaminergic cell death are of the utmost importance to identify new drug targets for the treatment of PD.

Oxidative toxicity in neurodegenerative diseases: role of mitochondrial dysfunction and therapeutic strategies

Besides fluorine, oxygen is the most electronegative element with the highest reduction potential in biological systems. Metabolic pathways in mammalian cells utilize oxygen as the ultimate oxidizing agent to harvest free energy. They are very efficient, but not without risk of generating various oxygen radicals. These cells have good antioxidative defense mechanisms to neutralize these radicals and prevent oxidative stress. However, increased oxidative stress results in oxidative modifications in lipid, protein, and nucleic acids, leading to mitochondrial dysfunction and cell death. Oxidative stress and mitochondrial dysfunction have been implicated in many neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, and stroke-related brain damage. Research has indicated mitochondria play a central role in cell suicide. An increase in oxidative stress causes mitochondrial dysfunction, leading to more production of reactive oxygen species and eventually mitochondrial membrane permeabilization. Once the mitochondria are destabilized, cells are destined to commit suicide. Therefore, antioxidative agents alone are not sufficient to protect neuronal loss in many neurodegenerative diseases. Combinatorial treatment with antioxidative agents could stabilize mitochondria and may be the most suitable strategy to prevent neuronal loss. This review discusses recent work related to oxidative toxicity in the central nervous system and strategies to treat neurodegenerative diseases.

Mechanism of pyrethroid pesticide-induced apoptosis: role of calpain and the ER stress pathway

Exposure to the pyrethroid pesticide deltamethrin has been demonstrated to cause apoptosis both in vitro and in vivo. However, the molecular pathways leading to deltamethrin-induced apoptosis have not been established. To identify these pathways, SK-N-AS neuroblastoma cells were exposed to deltamethrin (100 nM-5 μM) for 24-48 h. Deltamethrin produced a time- and dose-dependent increase (21-300%) in DNA fragmentation, an indicator of apoptosis. Data demonstrate that the initiation of DNA fragmentation resulted from interaction of deltamethrin with Na⁺ channels and consequent calcium influx, as tetrodotoxin and the intracellular Ca²⁺ chelator BAPTA-AM completely prevented apoptosis. DNA fragmentation was accompanied by increased caspase-9 and -3 activities and was abolished by specific caspase-9 and -3 inhibitors. However, deltamethrin did not increase cytosolic cytochrome c levels, indicating that the mitochondrial pathway was likely not involved. Additional studies demonstrated that deltamethrin exposure activated caspase-12 activity and that pharmacological inhibition and siRNA knockdown of calpain prevented deltamethrin-induced DNA fragmentation, thus indicating a role for the endoplasmic reticulum (ER) stress pathway. This was confirmed by the observation that inhibition of eIF2α abolished deltamethrin-induced DNA fragmentation. Together, these data demonstrate that deltamethrin causes apoptosis through its interaction with Na⁺ channels, leading to calcium overload and activation of the ER stress pathway. Because ER stress and the subsequent unfolded protein response have been observed in a number of neurodegenerative diseases, these data provide mechanistic information by which high-level exposure to pyrethroids may contribute to neurodegeneration.

Polygalae radix inhibits toxin-induced neuronal death in the Parkinson's disease models

AIM OF THE STUDY

Polygalae radix, the root of Polygala tenuifolia Willd, has commonly been used for the treatment of amnesia and anxiety in traditional Korean medicine. The aim of this study was to investigate its neuroprotective effects and possible mechanisms of action in models of Parkinson's disease.

MATERIALS AND METHODS

This study utilized a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, a reactive oxygen species (ROS) assay, a nitric oxide (NO) production assay, and a caspase-3 activity test as measures of cell viability in PC12 cells damaged by 6-hydroxydopamine (6-OHDA). The protective effects of PRE against 1-methyl-4-phenylpyridium (MPP(+))-induced neurotoxicity were assessed in rat primary dopaminergic neurons and in a mouse PD model in which PRE was administered (100mg/kg/day, 3 days, p.o.) before acute 1-mehtyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity. Finally, TH immunohistochemistry tests were conducted in the substantia nigra pars compacta (SNpc) and striatum (ST).

RESULTS AND CONCLUSIONS

PRE significantly inhibited 6-OHDA-induced cell damage at doses of 0.05-1μg/ml with a maximal effect at 0.1μg/ml. Caspase-3 activity and the production of ROS and NO were alleviated at 0.1μg/ml. Also at this dose, PRE protected mesencephalic dopaminergic neurons from MPP(+)-induced toxicity. In an in vivo mouse model of PD, PRE protected dopaminergic neurons and fibers from MPTP-induced toxicity in the SNpc and ST. These results demonstrate that PRE has protective effects on dopaminergic neurons via its anti-oxidant and anti-apoptotic activity.

DJ-1 deficiency in astrocytes selectively enhances mitochondrial Complex I inhibitor-induced neurotoxicity

Parkinson's disease (PD) brains show evidence of mitochondrial respiratory Complex I deficiency, oxidative stress, and neuronal death. Complex I-inhibiting neurotoxins, such as the pesticide rotenone, cause neuronal death and parkinsonism in animal models. We have previously shown that DJ-1 over-expression in astrocytes augments their capacity to protect neurons against rotenone, that DJ-1 knock-down impairs astrocyte-mediated neuroprotection against rotenone, and that each process involves astrocyte-released factors. To further investigate the mechanism behind these findings, we developed a high-throughput, plate-based bioassay that can be used to assess how genetic manipulations in astrocytes affect their ability to protect co-cultured neurons. We used this bioassay to show that DJ-1 deficiency-induced impairments in astrocyte-mediated neuroprotection occur solely in the presence of pesticides that inhibit Complex I (rotenone, pyridaben, fenazaquin, and fenpyroximate); not with agents that inhibit Complexes II-V, that primarily induce oxidative stress, or that inhibit the proteasome. This is a potentially PD-relevant finding because pesticide exposure is epidemiologically-linked with an increased risk for PD. Further investigations into our model suggested that astrocytic GSH and heme oxygenase-1 antioxidant systems are not central to the neuroprotective mechanism.

Mitochondrial Dysfunction: The Road to Alpha-Synuclein Oligomerization in PD

While the etiology of Parkinson's disease remains largely elusive, there is accumulating evidence suggesting that mitochondrial dysfunction occurs prior to the onset of symptoms in Parkinson's disease. Mitochondria are remarkably primed to play a vital role in neuronal cell survival since they are key regulators of energy metabolism (as ATP producers), of intracellular calcium homeostasis, of NAD(+)/NADH ratio, and of endogenous reactive oxygen species production and programmed cell death. In this paper, we focus on mitochondrial dysfunction-mediated alpha-synuclein aggregation. We highlight some of the findings that provide proof of evidence for a mitochondrial metabolism control in Parkinson's disease, namely, mitochondrial regulation of microtubule-dependent cellular traffic and autophagic lysosomal pathway. The knowledge that microtubule alterations may lead to autophagic deficiency and may compromise the cellular degradation mechanisms that culminate in the progressive accumulation of aberrant protein aggregates shields new insights to the way we address Parkinson's disease. In line with this knowledge, an innovative window for new therapeutic strategies aimed to restore microtubule network may be unlocked.

Molecular mechanisms of pesticide-induced neurotoxicity: Relevance to Parkinson's disease

Pesticides are widely used in agricultural and other settings, resulting in continued human exposure. Pesticide toxicity has been clearly demonstrated to alter a variety of neurological functions. Particularly, there is strong evidence suggesting that pesticide exposure predisposes to neurodegenerative diseases. Epidemiological data have suggested a relationship between pesticide exposure and brain neurodegeneration. However, an increasing debate has aroused regarding this issue. Paraquat is a highly toxic quaternary nitrogen herbicide which has been largely studied as a model for Parkinson's disease providing valuable insight into the molecular mechanisms involved in the toxic effects of pesticides and their role in the progression of neurodegenerative diseases. In this work, we review the molecular mechanisms involved in the neurotoxic action of pesticides, with emphasis on the mechanisms associated with the induction of neuronal cell death by paraquat as a model for Parkinsonian neurodegeneration.

Role of epigenetics in Alzheimer’s and Parkinson’s disease

Alzheimer’s disease (AD) and Parkinson’s disease (PD) are two common neurodegenerative diseases that result in the progressive damage or death of neurons. Environmental agents have the potential to damage the developing and mature nervous system, resulting in neurodegenerative diseases. Heritable changes in gene expression that do not involve coding sequence modifications are referred to as ‘epigenetic’. These modifications include DNA methylation and downstream modification of histones. Environmental factors, including heavy metals and dietary folate intake, perturb neurodegenerative genes by epigenetic means, leading to altered gene expression and late-onset neurodegenerative diseases. Research into the genetic control of DNA methylation indicates an allelic skewing in a significant proportion of genes. This phenomenon may determine how an individual’s genetic makeup can alter the effect an environmental factor has on their risk of developing neurodegeneration. Finally, preliminary evidence using cell culture and transgenic animal models suggests that whole classes of pan-epigenetic modifiers will have significant protective effects against common neurodegenerative diseases.

Parkinson's disease: is it a toxic syndrome?

Parkinson's disease (PD) is one of the neurodegenerative diseases which we can by certainty identify its pathology, however, this confidence disappeares when talking about the cause. A long history of trials, suggestions, and theories tried linking PD to a specific causation. In this paper, a new suggestion is trying to find its way, could it be toxicology? Can we—in the future—look to PD as an occupational disease, in fact, many clues point to the possible toxic responsibility—either total or partial—in causing this disease. Searching for possible toxic causes for PD would help in designing perfect toxic models in animals.

The Drosophila vesicular monoamine transporter reduces pesticide-induced loss of dopaminergic neurons

Dopamine is cytotoxic and may play a role in the development of Parkinson's disease. However, its interaction with environmental risk factors such as pesticides remains poorly understood. The vesicular monoamine transporter (VMAT) regulates intracellular dopamine content, and we have tested the neuroprotective effects of VMAT in vivo using the model organism Drosophila melanogaster. We find that Drosophila VMAT (dVMAT) mutants contain fewer dopaminergic neurons than wild type, consistent with a developmental effect, and that dopaminergic cell loss in the mutant is exacerbated by the pesticides rotenone and paraquat. Overexpression of DVMAT protein does not increase the survival of animals exposed to rotenone, but blocks the loss of dopaminergic neurons caused by this pesticide. These results are the first to demonstrate an interaction between a VMAT and pesticides in vivo, and provide an important model to investigate the mechanisms by which pesticides and cellular DA may interact to kill dopaminergic cells.

Extracellular redox status regulates Nrf2 activation through mitochondrial reactive oxygen species

The redox status of the extracellular compartment has only just been elucidated as a mechanism controlling intracellular signal transduction and correlates with aging, diabetes, heart disease and lung fibrosis. In the present paper, we describe a mechanism by which oxidizing extracellular environments, as maintained by the cysteine/cystine (Cys/CySS) redox couple, induce mitochondria-derived ROS (reactive oxygen species) generation and cause the activation of Nrf2 (nuclear factor-erythroid 2-related factor 2), inducing an antioxidant response. NIH 3T3 cells were cultured in medium with extracellular Cys/CySS redox potentials (Eh), ranging from 0 to -150 mV. Cellular and mitochondrial ROS production significantly increased in cells incubated under more oxidizing extracellular conditions (0 and -46 mV). Trx2 (thioredoxin-2) is a mitochondrial-specific oxidoreductase and antioxidant and became oxidized in cells incubated at 0 or -46 mV. MEFs (mouse embryonic fibroblasts) from Trx2-overexpressing transgenic (Trx2 Tg) mice produced less intracellular ROS compared with WT (wild-type) MEFs at the more oxidizing extracellular conditions. Nrf2 activity was increased in WT MEFs at the 0 or -46 mV conditions, but was inhibited in Trx2 Tg MEFs under the same conditions. Furthermore, Nrf2-regulated gene expression was significantly increased in the WT MEFs, but not in the Trx2 Tg MEFs. These results show that the Cys/CySS redox status in the extracellular compartment regulates intracellular ROS generated primarily in the mitochondria, which play an important role in the activation of Nrf2 and up-regulation of antioxidant and detoxification systems.

Preparation of methacrylic acid-modified rice husk improved by an experimental design and application for paraquat adsorption

Methacrylic acid (MAA) grafted rice husk was synthesized using graft copolymerization with Fenton's reagent as the redox initiator and applied to the adsorption of paraquat. The highest grafting percentage of 44.3% was obtained using the traditional kinetic method. However, a maximum grafting percentage of 65.3% was calculated using the central composite design. Experimental results based on the recipes predicted from the statistical analysis are consistent with theoretical calculations. A representative polymethacrylic acid-g-rice husk (PMAA-g-rice husk) copolymer was hydrolyzed to a salt type and applied to the adsorption of paraquat. The adsorption equilibrium data correlate more closely with the Langmuir isotherm than with the Freundlich equation. The maximum adsorption capacity of modified rice husk is 292.5mg/g-adsorbent. This value exceeds those for Fuller's earth and activated carbon, which are the most common binding agents used for paraquat. The samples at various stages were characterized by solid-state (13)C NMR spectroscopy.

Effects of insulin-like growth factor-1 on rotenone-induced apoptosis in human lymphocyte cells

Human peripheral blood lymphocytes have been useful as a putative model of oxidative stress-induced apoptosis for Parkinson's disease. The present work shows that rotenone, a mitochondrial complex I inhibitor, induced time- and concentration-dependent apoptosis in lymphocytes which was mediated by anion superoxide radicals (O(2)*(-))/hydrogen peroxide, depolarization of mitochondria, caspase-3 activation, concomitantly with the nuclear translocation of transcription factors such as NF-kappaB, p53, c-Jun and nuclei fragmentation. Since insulin-like growth factor-1 (IGF-1) interferes with a cell's apoptotic machinery when subjected to several stressful conditions, it is demonstrated here for the first time that IGF-1 effectively protects lymphocytes against rotenone through PI-3K/Akt activation, down-regulation of p53 and maintenance of mitochondrial membrane potential independently of ROS generation. These data might contribute to understanding the role played by IGF-1 against oxidative stress stimuli.

Identification of glutathione S-transferase pi as a protein involved in Parkinson disease progression

Parkinson disease (PD) typically affects the cortical regions during the later stages of disease, with neuronal loss, gliosis, and formation of diffuse cortical Lewy bodies in a significant portion of patients with dementia. To identify novel proteins involved in PD progression, we prepared synaptosomal fractions from the frontal cortices of pathologically verified PD patients at different stages along with age-matched controls. Protein expression profiles were compared using a robust quantitative proteomic technique. Approximately 100 proteins displayed significant differences in their relative abundances between PD patients at various stages and controls; three of these proteins were validated using independent techniques. One of the confirmed proteins, glutathione S-transferase Pi, was further investigated in cellular models of PD, demonstrating that its level was intimately associated with several critical cellular processes that are directly related to neurodegeneration in PD. These results have, for the first time, suggested that the levels of glutathione S-transferase Pi may play an important role in modulating the progression of PD.

MPTP intoxication in mice: a useful model of Leigh syndrome to study mitochondrial diseases in childhood

The basal ganglia, which are interconnected in the striato-nigral dopaminergic network, are affected in several childhood diseases including Leigh syndrome (LS). LS is the most common mitochondrial disorder affecting children and usually arise from inhibition of the respiratory chain. This vulnerability is attributed to a particular susceptibility to energetic stress, with mitochondrial inhibition as a common pathogenic pathway. In this study we developed a LS model for neuroprotection trials in mice by using the complex I inhibitor MPTP. We first verified that MPTP significantly inhibits the mitochondrial complex I in the brain (p = 0.018). This model also reproduced the biochemical and pathological features of LS: MPTP increased plasmatic lactate levels (p = 0.023) and triggered basal ganglia degeneration, as evaluated through dopamine transporter (DAT) autoradiography, tyrosine hydroxylase (TH) immunohistochemistry, and dopamine dosage. Striatal DAT levels were markedly decreased after MPTP treatment (p = 0.003). TH immunoreactivity was reduced in the striatum and substantia nigra (p = 0.005), and striatal dopamine was significantly reduced (p < 0.01). Taken together, these results confirm that acute MPTP intoxication in young mice provides a reproducible pharmacological paradigm of LS, thus opening new avenues for neuroprotection research.

Parkinson-linked genes and toxins that affect neuronal cell death through the Bcl-2 family

Parkinson's disease (PD) results from the death of specific neuronal populations in the CNS. Potential causative factors include environmental toxins and gene mutations that can combine to dysregulate the processing and degradation of alpha-synuclein. Oxidative stress induced by the neurotoxins MPTP, paraquat, maneb, and rotenone causes lipid peroxidation and protein misfolding that affects cell death through members of the Bcl-2 family. Sufficient activation of Bax and Bak facilitates mitochondrial outer-membrane permeabilization, which releases death-inducing factors that cause apoptotic and nonapoptotic programmed cell death. The formation of alpha-synuclein aggregates is a defining pathologic feature of PD and is induced by these neurotoxins as well as several Parkinson-linked familial mutations. Of the familial mutations identified thus far, two of the loci encode proteins associated with ubiquitin-proteasome degradation of misfolded proteins (Parkin and Uch-L1), and two encode proteins associated with mitochondria and oxidative stress (DJ-1 and PINK1). Both gene and toxin findings indicate that dopaminergic neuron losses in PD are the result of oxidative stress affecting mitochondria function and ubiquitin-proteasome activity. Here we describe how related cell death mechanisms are involved in the pathophysiology of Parkinson's disease.

Mitochondrial dysfunction in the limelight of Parkinson's disease pathogenesis

Parkinson's disease (PD) is a progressive neurodegenerative movement disorder with unknown etiology. It is marked by widespread neurodegeneration in the brain with profound loss of A9 midbrain dopaminergic neurons in substantia nigra pars compacta. Several theories of biochemical abnormalities have been linked to pathogenesis of PD of which mitochondrial dysfunction due to an impairment of mitochondrial complex I and subsequent oxidative stress seems to take the center stage in experimental models of PD and in postmortem tissues of sporadic forms of illness. Recent identification of specific gene mutations and their influence on mitochondrial functions has further reinforced the relevance of mitochondrial abnormalities in disease pathogenesis. In both sporadic and familial forms of PD abnormal mitochondrial paradigms associated with disease include impaired functioning of the mitochondrial electron transport chain, aging associated damage to mitochondrial DNA, impaired calcium buffering, and anomalies in mitochondrial morphology and dynamics. Here we provide an overview of specific mitochondrial functions affected in sporadic and familial PD that play a role in disease pathogenesis. We propose to utilize these gained insights to further streamline and focus the research to better understand mitochondria's role in disease development and exploit potential mitochondrial targets for therapeutic interventions in PD pathogenesis.

Role of protein kinase Cdelta in paraquat-induced glial cell death

Paraquat (1,1'-dimethyl-4,4'-bipyridinium) is structurally similar to the neurotoxin 1-methyl-4-phenyl-4-phenylpyridium ion (MPP+), the active metabolite of the parkinsonism-inducing agent 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which can induce the parkinsonism property in rodents, nonhuman primates, and human. In contrast to the neurotoxic effects of paraquat, little is known about its effects on glial cells. Here, we examined the mechanisms of paraquat toxicity in glial cells in culture. Paraquat treatment also reduced the viability of C6 glial cells in primary astrocyte cultures, and cell death was mostly apoptotic in nature. PKCdelta played a central role in the paraquat-induced glial cell death: (1) the PKCdelta-specific inhibitor rottlerin blocked paraquat-induced glial cell death; (2) paraquat induced tyrosine and threonine phosphorylation of PKCdelta; and (3) transfection of the dominant-negative mutant of PKCdelta attenuated paraquat toxicity. PKCdelta was also involved in the generation of reactive oxygen species (ROS), which mediated the paraquat toxicity. The nicotinamide adenine dinucleotide phosphate (reduced form) oxidase (NADPH oxidase) inhibitor diphenyleneiodonium blocked the paraquat-induced ROS production and subsequent cell death, indicating the involvement of NADPH oxidase in the cytotoxic action of paraquat in glia. PKCdelta was also important in glial cell death induced by MPP+ but not in that induced by rotenone. Last, Rac1 appeared to antagonize paraquat toxicity in glia. These results indicate a gliotoxic effect of paraquat and an opposing role of PKCdelta and Rac1 in paraquat-induced glial cell death.

Disruption of dopamine transport by DDT and its metabolites

Epidemiological studies suggest a link between pesticide exposure and an increased risk of developing Parkinson's disease (PD). Although studies have been unable to clearly identify specific pesticides that contribute to PD, a few human studies have reported higher levels of the organochlorine pesticides dieldrin and DDE (a metabolite of DDT) in post-mortem PD brains. Previously, we found that exposure of mice to dieldrin caused perturbations in the nigrostriatal dopamine system consistent with those seen in PD. Given the concern over the environmental persistence and reintroduction of DDT for the control of malaria-carrying mosquitoes and other pests, we sought to determine whether DDT and its two major metabolites, DDD and DDE, could damage the dopamine system. In vitro analyses in mouse synaptosomes and vesicles demonstrated that DDT and its metabolites inhibit the plasma membrane dopamine transporter (DAT) and the vesicular monoamine transporter (VMAT2). However, exposure of mice to either DDT or DDE failed to show evidence of nigrostriatal damage or behavioral abnormalities in any of the measures examined. Thus, we report that in vitro effects of DDT and its metabolites on components of the dopamine system do not translate into neurotoxicological outcomes in orally exposed mice and DDT appears to have less dopamine toxicity when compared to dieldrin. These data suggest elevated DDE levels in PD patients may represent a measure of general pesticide exposure and that other pesticides may be responsible for the association between pesticide exposure and PD.

Nmnat delays axonal degeneration caused by mitochondrial and oxidative stress

Axonal degeneration is a prominent feature of many neurological disorders that are associated with mitochondrial dysfunction, including Parkinson's disease, motor neuron disease, and inherited peripheral neuropathies. Studies of the Wld(s) mutant mouse, which undergoes delayed Wallerian degeneration in response to axonal injury, suggest that axonal degeneration is an active process. Wld(s) mice also have slower axonal degeneration and disease progression in numerous models of neurodegenerative disease. The Wld(s) mutation results in the production of a chimeric protein that contains the full-length coding sequence of nicotinamide mononucleotide adenylyltransferase 1 (Nmnat1), which alone is sufficient for axonal protection in vitro. To test the effects of increased Nmnat expression on axonal degeneration induced by mitochondrial dysfunction, we examined dorsal root ganglion (DRG) neurons treated with rotenone. Rotenone induced profound axonal degeneration in DRG neurons; however, this degeneration was delayed by expression of Nmnat. Nmnat-mediated protection resulted in decreased axonal accumulation and sensitivity to reactive oxygen species (ROS) but did not affect the change in the rate of rotenone-induced loss in neuronal ATP. Nmnat also prevented axonal degeneration caused by exposure to exogenous oxidants and reduced the level of axonal ROS after treatment with vincristine, further supporting the idea that Nmnat promotes axonal protection by mitigating the effects of ROS.

Parkinson's disease and pesticides: a toxicological perspective

Environmental factors have been shown to contribute to the incidence of Parkinson's disease (PD). Pesticides, which represent one of the primary classes of environmental agents associated with PD, share the common feature of being intentionally released into the environment to control or eliminate pests. Pesticides consist of multiple classes and subclasses of insecticides, herbicides, rodenticides, fungicides, fumigants and others and exhibit a vast array of chemically diverse structures. In this review we examine the evidence regarding the ability of each of the major pesticide subclasses to increase the incidence of PD. We propose that, from a toxicological perspective, it would be beneficial to identify specific subclasses, common structural features and the propensity for widespread human exposure when considering the potential role in PD, rather than using the overly broad term of 'pesticides' to describe this diverse group of chemicals. Furthermore, these chemicals and their environmentally relevant combinations should be evaluated for their ability to promote or accelerate PD and not merely for being singular causative agents.

Nonequilibrium thermodynamics of thiol/disulfide redox systems: a perspective on redox systems biology

Understanding the dynamics of redox elements in biologic systems remains a major challenge for redox signaling and oxidative stress research. Central redox elements include evolutionarily conserved subsets of cysteines and methionines of proteins which function as sulfur switches and labile reactive oxygen species (ROS) and reactive nitrogen species (RNS) which function in redox signaling. The sulfur switches depend on redox environments in which rates of oxidation are balanced with rates of reduction through the thioredoxins, glutathione/glutathione disulfide, and cysteine/cystine redox couples. These central couples, which we term redox control nodes, are maintained at stable but nonequilibrium steady states, are largely independently regulated in different subcellular compartments, and are quasi-independent from each other within compartments. Disruption of the redox control nodes can differentially affect sulfur switches, thereby creating a diversity of oxidative stress responses. Systems biology provides approaches to address the complexity of these responses. In the present review, we summarize thiol/disulfide pathway, redox potential, and rate information as a basis for kinetic modeling of sulfur switches. The summary identifies gaps in knowledge especially related to redox communication between compartments, definition of redox pathways, and discrimination between types of sulfur switches. A formulation for kinetic modeling of GSH/GSSG redox control indicates that systems biology could encourage novel therapeutic approaches to protect against oxidative stress by identifying specific redox-sensitive sites which could be targeted for intervention.

Protective effects of a new metalloporphyrin on paraquat-induced oxidative stress and apoptosis in N27 cells

Paraquat (PQ, 1,1'-dimethyl-4,4'-bipyridinium), a widely-used herbicide, has been suggested as a potential etiologic factor for the development of Parkinson's disease. In recent years, many studies have focused on the mechanism(s) of PQ neurotoxicity. In this study, we examined the neuroprotective effect of manganese (III) meso-tetrakis (N,N'-diethylimidazolium) porphyrin (MnTDM), a superoxide dismutase/catalase mimetic, on PQ-induced oxidative stress and apoptosis in 1RB3AN27 (N27) cells, a dopaminergic neuronal cell line. The results indicated that MnTDM significantly attenuated PQ-induced loss of cell viability, glutathione depletion, and reactive oxygen species production. MnTDM also ameliorated PQ-induced morphological nuclear changes of apoptosis and increased rates of apoptosis. In addition, our data provide direct evidence that MnTDM suppressed PQ-induced caspase-3 cleavage, possibly a key event of PQ neurotoxicity. These observations suggested that oxidative stress and apoptosis are implicated in PQ-induced neurotoxicity and this toxicity could be prevented by MnTDM. These findings also proposed a novel therapeutic approach for Parkinson's disease and other disorders associated with oxidative stress.

Redox compartmentalization in eukaryotic cells

Diverse functions of eukaryotic cells are optimized by organization of compatible chemistries into distinct compartments defined by the structures of lipid-containing membranes, multiprotein complexes and oligomeric structures of saccharides and nucleic acids. This structural and chemical organization is coordinated, in part, through cysteine residues of proteins which undergo reversible oxidation-reduction and serve as chemical/structural transducing elements. The central thiol/disulfide redox couples, thioredoxin-1, thioredoxin-2, GSH/GSSG and cysteine/cystine (Cys/CySS), are not in equilibrium with each other and are maintained at distinct, non-equilibrium potentials in mitochondria, nuclei, the secretory pathway and the extracellular space. Mitochondria contain the most reducing compartment, have the highest rates of electron transfer and are highly sensitive to oxidation. Nuclei also have more reduced redox potentials but are relatively resistant to oxidation. The secretory pathway contains oxidative systems which introduce disulfides into proteins for export. The cytoplasm contains few metabolic oxidases and this maintains an environment for redox signaling dependent upon NADPH oxidases and NO synthases. Extracellular compartments are maintained at stable oxidizing potentials. Controlled changes in cytoplasmic GSH/GSSG redox potential are associated with functional state, varying with proliferation, differentiation and apoptosis. Variation in extracellular Cys/CySS redox potential is also associated with proliferation, cell adhesion and apoptosis. Thus, cellular redox biology is inseparable from redox compartmentalization. Further elucidation of the redox control networks within compartments will improve the mechanistic understanding of cell functions and their disruption in disease.

Missense mutations of dual oxidase 2 (DUOX2) implicated in congenital hypothyroidism have impaired trafficking in cells reconstituted with DUOX2 maturation factor

Dual oxidase 2 (DUOX2), a reduced NAD phosphate:O2 oxidoreductase flavoprotein, is a component of the thyrocyte H2O2 generator required for hormone synthesis at the apical plasma membrane. We recently identified a specific DUOX2 maturation factor (DUOXA2) that is necessary and sufficient for expression of functional DUOX2 in mammalian cell lines. We have now used a DUOXA2 reconstituted system to provide the first characterization of natural DUOX2 missense variants (Q36H, R376W, D506N) at the molecular level, analyzing their impact on H2O2 generation, trafficking, stability, folding, and DUOXA2 interaction. The Q36H and R376W mutations completely prevent routing of DUOX2 to the cell surface. The mutant proteins are predominantly present as core N-glycosylated, thiol-reduced folding intermediates, which are retained by the quality control system within the endoplasmic reticulum (ER) as indicated by increased complexation with the lectin calnexin. D506N displays a partial deficiency phenotype with reduced surface expression of a mutant protein with normal intrinsic activity in generating H2O2. D506N N-glycan moieties are not subject to normal modification in the Golgi apparatus, suggesting that nonnative protein can escape the quality control in the ER. Oxidative folding of DUOX2 in the ER appears to be the rate-limiting step in the maturation of DUOX2, but is not facilitated by DUOXA2. Rather, DUOXA2 allows rapid ER exit of folded DUOX2 or enhanced degradation of mutant DUOX2 proteins not competent for ER exit. DUOXA2 may thus be part of a secondary quality control system specific for DUOX2.